question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>Diffraction can be observed in physical waves very easily, however when it comes to the diffraction of electromagnetic waves in things like the single slit experiment, I become a little confused. In things like water, it seems like there is something "holding together" the wave, although I'm not sure what it is. In the case of electromagnetic waves, why does diffraction occur?
I know it works out when viewed via Huygens' principle, but what is the mechanism behind electromagnetic diffraction? </p> | g14353 | [
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<blockquote>
<p>The frequency applied to a circuit of voltage 120 V with a <strong>real</strong> coil and a resistor has a value of 50 Hz. The resistance of the resistor is 10 $\Omega$. The voltage at the resistor terminals $u_1=60V$. The voltage at the coil's terminals is $u_2=90V$.</p>
</blockquote>
<p>$$\nu=50\text{ Hz}$$<br>
$$R=10\ {\Omega }$$
$$U=120\text{ V}$$<br>
$$u_1=60\text{ V - resistor terminal voltage}$$<br>
$$u_2=90\text{ V - coil terminal voltage}$$</p>
<blockquote>
<p>Find:</p>
<ul>
<li>the intensity of the current $I$.</li>
<li>the parameters of the coil.</li>
</ul>
</blockquote>
<p>I think by "parameters of the coil", it is meant the resistance and the impedance of the coil. </p>
<p>$$I=?$$<br>
$$L=?$$<br>
$$R_L=?$$ </p>
<p>I've been trying a bit, but I am quite poor at physics. This problem is suggested in a book, I want to prepare for a testpaper. </p>
<p>What I've been thinking of is to calculate $$\cos\phi=\frac{U_r }{U}=\frac{60}{120}=\frac{1}{2}$$ $$\implies \phi=\pi/3$$ But I am not sure if the resistor terminal voltage is the same thing with $U_r$. Is this right?</p> | g14354 | [
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<p>As far as I've seen, E&M waves are sinusoidal. Our eyes observe these patterns and give us colors, but in a situation like such: <a href="http://physics.stackexchange.com/questions/70054/why-do-electrons-emit-radiation?noredirect=1#comment142793_70054">Why do electrons emit radiation?</a>
There is still a wave, or at least a pulse, but it's not necessarily sinusoidal, and I'm not sure what 'wavelength' would be used. Nonetheless, radiation produced in this way does have a wavelength-it is used in synchrotrons. So what am I missing?</p> | g14355 | [
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<p>The equation for the double slit experiment requires an approximation in naming the two angles used to both be theta, correct? I feel like the majority of equations are exact relations, and it just seems strange that the equation for calculating the maxima an minima doesn't have an exact form.
Is the solution of d*sin(theta)=m*lambda really just an approximation. I know that it <em>is</em> a very accurate one, given the size of wavelengths, however it just seems so strange to be using an approximation, and to never see the <em>real</em> solution.</p> | g14356 | [
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<p>According to the Pauli principle, there can be no more than two electrons in a given state. If there are a number N of electrons in this box in the lowest states possible, show that the energy of the "top" electron - the "Fermi energy" $E_F$ is $E_F$ = $(2$$\pi$$\hslash$$N)$$/$$2mA$ , where $A$ $=$ $L^2$ and $L$ is the length of the square box the electrons are in. Now what I am thinking of using is ($E_{n_x n_y}$)$=(n_x^2+n_y^2)($$\pi^2$$\hslash$$)/$($2mL^2)$. Any hints on proceeding to next steps?</p> | g14357 | [
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<p>Why does light continue on forever if it was created from some source whose radiation dwindles at a rate of the inverse square of distance. Clearly light can be viewed as an interdependent phenomena, the E field pulling the B field along with it, but if all light must come from a source, and that source creates a field dependent on distance, why doesn't the light die off?</p> | g14358 | [
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<p>I am in trouble with calculation details of <a href="http://arxiv.org/abs/hep-th/9204083" rel="nofollow">Witten's Two dimensional Gauge Theories Revisited</a>. My questions is about (3.21) and (3.27).
From section 3, we have
$$\delta A_i=i\epsilon \psi_i\\
\delta \psi_i=-\epsilon D_i\phi \\
\delta \phi=0\\
\delta \lambda=i\epsilon\eta\\
\delta\eta=\epsilon[\phi,\lambda]\\
\delta \chi=\epsilon H\\
\delta H=i\epsilon [\phi,\chi]\\
V= \frac{1}{h^2}\int_\Sigma d\mu {\rm{Tr}} (\frac{1}{2}\chi(H-2\star F)+g^{ij}D_i\lambda\psi_j) $$
Then the (3.21) is given by
$$ L=-i\{Q,V \}\\= \frac{1}{h^2}\int_\Sigma d\mu {\rm{Tr}}(\frac{1}{2}(H-f)^2-\frac{1}{2}f^2 -i\chi\star D\psi+iD_i\eta\psi^i +D_i\lambda D^i\phi +\frac{i}{2}\chi[\chi,\phi]+i[\psi_i,\lambda]\psi^i ) $$</p>
<p>My questions are followings:</p>
<p>1) What is the relations between $\delta \bullet$ and $\{Q,\bullet\}$? Is there a minus sign when Q-operator crossing $\lambda$ or $\chi$ ? I always found different signs in terms $ -i\chi\star D\psi$ and $ D_i\lambda D^i\phi$ when I use relation $\delta\bullet=-i\epsilon\{Q,\bullet \}$.</p>
<p>2) Why the two terms $ -i\chi\star D\psi$ and $ D_i\lambda D^i\phi$ have different signs in (3.21) and (3.27)? Anything wrong with it?</p>
<p>======================================================================== =====</p>
<p>Based on discussion with Trimok, (Thanks to Trimok but this does not mean Trimok agrees with my views) I think I understand part of the problem. First, the relations is $\delta\bullet=-i\epsilon\{Q,\bullet \}$. $\delta$ pass $\chi$ and $\eta$ will give a minus sign. So in the formula (3.17), some of terms should be $ i\chi\star D\psi-D_i\lambda D^i\phi$ and (3.27) is correct. While I have a new question now about (3.28).</p>
<p>From (3.27) to (3.28), the procedures are the followings:
First, perform variation of H, and then integrate out H.
Second, perform variation of f, and then integrate out $\lambda$.
Second, perform variation of $\eta$, and then integrate out $\chi$.
After these calculation, the last term in (3.28), i.e. $\frac{1}{t^2} \frac{1}{2} (-D_kD^k+i[\psi_k,\psi^k])^2 $ does not appear. It seems from variation of $f$ from first part of (3.28). How to produce such a term in integrating out fields?</p> | g14359 | [
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<p>I am currently studying a Bachelor of Technology in the subject "Food Technology". I have completed 2 years just now.Yes, I have been exposed to Mathematics in my first year of college studying Differential and Integral Calculus,Ordinary Differential Equations,Linear Algebra, Complex Numbers, Vector Calculus,Probability. As for Physics, I have been exposed to courses like Oscillations and Waves, Wave Nature of Light, Special Theory of Relativity, Wave Mechanics, Atomic Physics(typically in the first year). The courses were not so rigorous( I am into an average university). In the 2nd year, we had different physics subjects like Fluid Mechanics, Thermodynamics, Heat and Mass Transfer. I am now going to be exposed to more Food Subjects from the 3rd Year like Food Engineering and Meats,Poultry etc . This was only to inform you of my current status.</p>
<p>However, I am not really keen on pursuing Food Science in my Post Grad career. I cannot change my course of choice right now,unfortunately. However,for the Post Grad, I have majorly three to four options in mind, comprising of Maths, Physics, Mechanical Engineering or Computer Science. I know, not a clear goal yet. Out of these, I have been strongly inclining towards Maths/Physics at the moment. Doing Food Technology had never been my goal. Due to a lack of clear focus, I ended up getting bad grades and choosing a course, perhaps not so suitable for me. </p>
<p>I always had an intensely logical approach to everything. Whenever I study Mathematics, I want to delve down into the details of how a particular proof was created. I just lose interest in solving problems when I don't know all the details that went into creating it.I absolutely love the idea of constructing new mathematical models or creating new science, something never known before, not for the sake of getting 'famous', but for the sake for the unlimited intellectual stimulation and satisfaction it would provide me with.
Since my college courses weren't so rigorous, I nearly memorized a lot of things very unwillingly and had very good marks in some of the maths/physics subjects, and not so good in some other such subjects. I am truly fascinated by the idea of Astrophysics. Any physics or maths, that helps me understand how the world works, and especially would help me innovate or write a new model after having a thorough read of the earlier established work in "intricate" detail is the best for me. I think of "why's" to every possible maths or physics statement for a very long time. Yet as of today, due to no clear structure, I have not been able to have a firm grasp on the Physics or Maths Fundamentals. I reside in India, and it is not so research driven as countries like U.S.A. with the exception of some premier institutes(which unfortunately I didn't get into, because I was very unsure of what I wanted to do in my life).
I have decided I will finally work to the best of my abilities and get strong in the basics. I am not a genius, but I am smart. I believe I can do it. I have been looking at some of the top graduate schools for these programs like MIT,Princeton,Cal tech (you get the idea), but the lack of a proper structure and plan have always lead me in confusion. I know I will love very very intellectually stimulating places, looking at every piece of math or science work , and thinking what lead to this in extreme detail. Also, I would love a place where cutting edge research is done, creating the potential of developing several new innovations that change the future of math or science. </p>
<p>I have also considered the option of integrating food science and physics/maths/mechanical engineering in research to create something challenging and deeply meaningful. </p>
<p>Also,I have always been very very interested in the theoretical maths/physics rather than the applied(attending labs) parts. The lab courses just cannot hold my interest for long.</p>
<p>Now, I know, that on a scale of 1-10, I want to go to 10, and I am currently at 0-1. Would would you suggest I do? How should I prepare? How can I get into such top schools? How can I start at this particular moment?
I am open to all useful suggestions. Especially, from people who have experience in such fields. If I don't do this, I will regret not having done this for my whole life. I clearly need help, because I realize that if I don't have a great plan, I won't be able to achieve this. What are the 'essentials' I really need and after then,what next? Please take this into consideration. I have about 2 years of college left, what can I do?</p> | g14360 | [
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<p>I don't remember enough from my electromagnetism course and I can't find any simple, full example on this subject. </p>
<p>I know, that I can consider the cylinder as a wire with the same charge density (when outside of the cylinder), so I consider the wire case with the same charge density, $\lambda$.</p>
<p>I set the $y$ direction to pass through the wire, and the $x$ direction to pass through the point charge and perpendicular to the wire. Everything is in the $z=0$ plane. Hence, the particle is at $x=r$ position. </p>
<p>From <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecyl.html" rel="nofollow">Gauss law</a>, I learn that the field at a point at distance $r$ from the wire I have a field which scales like $\lambda/r$. To find the <a href="http://en.wikipedia.org/wiki/Electric_potential#In_electrostatics" rel="nofollow">potential</a>, I need to solve the integral </p>
<p>$V = -\int_b^r E dl = - k \int_b^r \lambda/x dx$</p>
<p>Where $k$ holds the constants and $b$ being a point where the potential is 0.</p>
<p>The solution of this integral is </p>
<p>$V = - k \lambda \log (1/r) + C$</p>
<p>When $b=1$ the potential is 0 so the potential is</p>
<p>$V = k \lambda \log (1/r)$</p>
<p>and the energy is </p>
<p>$U = k q \lambda \log (1/r)$ </p>
<p>Here are my questions:</p>
<ol>
<li><p>Is any of this true?</p></li>
<li><p>I tried to derive this by starting with coulomb law and calculating the energy for a segment of the wire, I get an answer which scales like $1/r$, can you derive the answer for this?</p></li>
</ol> | g14361 | [
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<p>I'm new to electronics. I have a doubt. That is...</p>
<p>Is watt unit depended on speed of turbine ?</p>
<p><strong>In my mind</strong></p>
<p>When turbine rotates fast the power in increase. Is it right ?</p>
<p>Please help me.</p>
<p>Thanks for your response...</p> | g14362 | [
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<h1>Overview</h1>
<p>Einstein's proof for the <a href="http://en.wikipedia.org/wiki/Lorentz_transformation" rel="nofollow">Lorentz transformation</a> is given <a href="http://en.wikipedia.org/wiki/Derivations_of_the_Lorentz_transformations#From_physical_principles" rel="nofollow">here</a>:</p>
<p>From $O$'s view point, $x^2+y^2+z^2 = (ct)^2$.</p>
<p>Form $O'$'s view point, $x'^2+y'^2+z'^2 = (ct')^2$. </p>
<p>We find that Einstein has imparted the information that the velocity of light observed by $O$ and $O'$ is constant (Einstein's first postulate).</p>
<h1>Problem statement</h1>
<p>All this has made me wonder why should a postulate derive his result? Could his results be derived by the existing laws of physics? Every textbook that I have referred says that his postulates are a direct application of Michelson-Morley experiment's results. But what actually causes this time dilation and length contraction? </p>
<h1>Update</h1>
<p>I haven't asked about why we define postulates. I understand that at one point we must have postulates. But to elaborate, my question is about: is it possible to <strong>derive</strong> Einstein's postulates form works of Maxwell?</p>
<p>Note: I know that a postulate does not have a derivation, but how do you think this postulate works? </p>
<p>To show you that Maxwell's laws have something to do with Einstein's postulates:</p>
<p>Consider 2 point charges moving with the same velocity parallel to each other. We observe that force acts on both the particles due to magnetic field. But, the observer moving along with the charge must observe no force at all! Don't you think Einstein's postulates can be derived from Maxwell's work? </p> | g14363 | [
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<p>Assume I have built up a pretty high charge by rubbing the floor or something. I want to understand these situations:</p>
<ol>
<li><p>I almost always get shocked when I touch a metal doorknob with my bare hand.</p></li>
<li><p>I occasionally get shocked if I touch the wooden door first, then touch the metal doorknob.</p></li>
<li><p>I almost never get shocked if I'm holding a metal key, and use that key to touch the doorknob. Sometimes I can even see sparks between the key and the doorknob.</p></li>
<li><p>This just happened, and I feel is really weird: I'm holding a metal travel mug (water bottle) full of water. I start pouring the water down to a metal sink. I get shocked exactly when the water hits the sinks.</p></li>
</ol> | g14364 | [
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<p>I'm trying to scale a physical object up in size, either double or quadruple. It's a circular acrylic plate, with multiple weights attached around it's edge, to lower the resonant frequency to a specific frequency, 32hz. </p>
<p>The plate is 16” in diameter, the weights around it's edge add up to 1.05 kg. It needs to be scaled to 32” or 64”. It will scale in diameter but not thickness, which will remain 1/8” for all diameters. </p>
<p>If the size is doubled to 32”, along with doubling the weights around the edge to 2.1 kg, will the resonant frequency be lowered to the same frequency as the 16” plate with 1.05 kg weights?</p>
<p>If not, how does this scale? How far off approximately will the resonant frequency be from 32hz?</p>
<p>Same question for scaling to 64” with 4.2 kg weights around the edge.</p> | g14365 | [
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<p>Suppose we have several antennae to transmit and receive radio waves. What should one transmit, and what kind of equations are used to compute $reflectivity(\vec{x})$ for points in space from a given discretized signal over several antennae?</p> | g14366 | [
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<p>Famously, the path integral of quantum field theory is related to the partition function of statistical mechanics via a Wick rotation and there is therefore a formal analogy between the two. I have a few questions about the relation between the two objects.</p>
<ol>
<li><p>Loop diagrams in quantum field theory have a nice interpretation in terms of virtual particles. What is the interpretation of loops in statistical mechanics?</p></li>
<li><p>Does the relation between the two objects imply that for every quantum mechanical phenomenon there's a corresponding statistical mechanics phenomenon and vice versa? If not, where does the analogy fail? </p></li>
<li><p>If so, what would be the phenomena analogous to the Aharonov-Bohm (AB) effect? It's hard for me to see the analog for any quantum effect that depends on phase interference like the AB effect. </p></li>
</ol> | g14367 | [
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<p>We know that a moving charge produces a magnetic field. But which frame of reference it must move to produce magnetic field?
And also if a charged particle moves in a magnetic field, magnetic force acts on it. In which frame of reference it must move to feel a force?</p> | g14368 | [
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<p>In my dynamics notes I have written the following:
$$\frac{d\vec{p}_A}{dt}= \sum\vec{AC_i}\times m\vec{a_{ci}} + \sum \frac{dR_i}{dt}\left\{{I^{(i)}_{ci}}{\omega}^{(i)}_{ci}\right\}+\sum R\left\{{I^{(i)}_{ci}}\frac{d{\omega}^{(i)}_{ci}}{dt}\right\}$$
where $A$ is a random point, $C_i$ the center of mass of object $i$, $\left\{{I^{(i)}_{ci}}{\omega}^{(i)}_{ci}\right\}$ (an admittedly strange notation for) the resulting vector from $I_{i}\vec{\omega_{ci}}$ if $I_{i}$ is a diagonal matrix and $R_i$ a matrix that projects object $i$ to its principal axes of inertia. I am, however, incapable of finding my derivation of this formula.</p>
<p>Is this formula always correct? And also, -if correct- could $\frac{dR_i}{dt}$ be replace with $\vec{\omega_i}\times R_i$?</p> | g14369 | [
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0.030289404094219208,
-... |
<p>I am trying to model an open system interaction without making strong assumptions on coupling strength or temperature. In general i understand that open systems are modeled by a Lindbladian, but as far as i know the Lindbladian approximation holds only if we have Marcov, Born and Circular Wave approximation. Since I want to cover a broad range of temperatures and coupling strengths how should i model the bath? Any suggestions on how to proceed?</p>
<p>More specificly a colleague suggested using the coupled harmonic oscillator formalism has the advantage of not making any assumptions apart from being modelable by harmonic oscillators and being analytical solvable! Anyone has recommendations where i can read up how this is done or an opinion if this approach is valid?</p> | g14370 | [
0.04223709553480148,
-0.03330063819885254,
0.007086045574396849,
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0.03926878795027733,
0.004288649186491966,
0.010974666103720665,
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-0.023894747719168663,
0.02111961878836155,
0.03317151218652725,
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0.0337... |
<p>The closest question I could find with regards to this subject was this one:<br>
<a href="http://physics.stackexchange.com/questions/20730/countersteering-a-motorcycle">Countersteering a motorcycle</a></p>
<p>However, it does not address the specific physics of what I would like to know.</p>
<p>There are 3 ways to lean when turning a motorcycle:</p>
<ol>
<li>Upper body remains upright while the bike leans.</li>
<li>Whole body remains aligned with bike.</li>
<li>Most of the body "hangs off" the side leaning in.</li>
</ol>
<p>I'm trying not to make any assumptions to allow for detailed and proper answers addressing issues I may not have considered; hopefully, without being too generic.</p>
<p>So to summarize, I would like to know whether the first 2 items are sufficient for all conditions or whether the 3rd has some physical properties necessary in certain conditions.</p> | g14371 | [
0.08354850858449936,
0.0028690840117633343,
0.005674953572452068,
0.038253117352724075,
0.04074345901608467,
0.06309395283460617,
0.04763571172952652,
0.035441700369119644,
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-0.037674229592084885,
0.023970838636159897,
-0.06898178160190582,
-0.022137213498353958,
-0.03... |
<p>I am learning aerodynamics. In this course a potential flow is denoted that a flow in which the rotation is zero everywhere. But the book told me that we can add vortex into a flow field, and we can also use potential theory to analysis it. I get confused about this. If a vortex is added in a flow field, I don't think there exist any potential. The rotation of a vortex is infinite! Could anyone explain to me, why potential flow theory can be used while there are some vortexes in the flow field?</p> | g14372 | [
0.03284119814634323,
0.04007374122738838,
0.019968274980783463,
-0.0010020107729360461,
0.05342414230108261,
0.02599399723112583,
0.004921543411910534,
0.017792874947190285,
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-0.04749296233057976,
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-0.009303135797381401,
-0.04968554899096489,
0.... |
<p>Like in the question. Why neodymium magnets (Nd2Fe14B) are called "neodymium magnets"? Why not boron magnets? Or iron magnets?</p> | g14373 | [
0.01413780078291893,
0.03761120140552521,
0.01929781585931778,
0.02299889549612999,
0.03352262079715729,
0.04727837070822716,
0.00046672430471517146,
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-0.022247115150094032,
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0.003430979559198022,
0.01... |
<p>A time-varying $B(t)$ field through a loop antenna induces a voltage proportional to $\dot{B}(t)$. A Hertzian dipole along a time-varying $E(t)$ field also induces a voltage across a load--while I haven't seen a rigorous derivation of what it is, since we have a zero output when $\dot{E}(t) = 0$, it seems likely to me that the output is proportional to $\dot{E}(t)$.</p>
<p>So I was wondering if this is true of <em>any</em> antenna? Since in radiation, at any point in the far field, the magnitudes $E(t) = cB(t)$, is it true the voltage across any resistive load at the receiver is proportional to $\dot{E}(t)$?</p>
<p><strong>If so</strong>, since in the far field $E(t) \propto \dot{I}(t_r)$, does this mean that the voltage across the load in the receiver is proportional to $\ddot{I}(t_r)$? And since this would mean that the power transferred to the load would depend on $\ddot{I}(t_r)$ as well, and we know that the Poynting vector depends on $\dot{I}(t_r)$, what happens to the difference? </p> | g14374 | [
0.002040516585111618,
-0.015374037437140942,
-0.023724481463432312,
-0.05430254712700844,
0.0532337985932827,
0.00196136930026114,
0.00740461191162467,
0.0018379292450845242,
-0.028562674298882484,
0.0069894008338451385,
-0.05923749506473541,
0.04582959786057472,
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-0.... |
<p>In a laser interferometry experiment, we project a pattern of interference fringes onto a CCD sensor. For best results, we want good contrast between the bright and dark fringes, and we carefully compensate for various sources of noise - for example, by taking camera images with no fringes present, and with the laser turned off, and subtracting these images in the proper sequence. We'd expect therefore that the remaining signal should be highly linear, with the CCD signal at each pixel in direct proportion to the number of photons reaching it during the shutter time. </p>
<p>What we actually find is that, as we vary the laser intensity and shutter time such that the average intensity across the image remains constant, with no pixels saturated, there is a definite "sweet spot" where the fringes are much more well-defined than at other settings. Either increasing or decreasing the laser intensity away from this point (with corresponding decreases or increases in shutter time) causes the fringe definition to deteriorate.</p>
<p>I can't think of any reason why this should be. I know that in cases where the process generating the pattern has a time-constant (for example, using laser speckle interferometry to measure Brownian motion), there is an optimum exposure setting, but that shouldn't be the case in our system which is entirely static. So, what am I missing? I assume it's some property of the CCD sensor that I've overlooked. </p> | g14375 | [
0.03391265124082565,
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0.019541479647159576,
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0.03512701019644737,
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0.023713143542408943,
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-0.05251481011509895,
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0.0238406453281641,
0.018278684467077255,
0.04997... |
<blockquote>
<p><em>A train rounds a curve of radius 235 m (turning right). The train track is flat, and the train is traveling at a constant speed. A lamp in the train makes an angle to the vertical of 17.5 degrees. How do you calculate the speed of the train?</em></p>
</blockquote>
<p>The first thing I need is to draw a free-body diagram of the lamp.<img src="http://i.stack.imgur.com/SQTWH.png" alt="free-body diagram of lamp"></p>
<p>Sorry about the bad drawing.
What confuses me is what force drives the lamp to the left. It seems to have something to do with inertia, I think it is probably equal to the centripetal force that the train exerts on anything in it. Can someone identify this force and explain the problem?</p> | g14376 | [
0.009454351849853992,
0.029972879216074944,
0.00007064880628604442,
-0.023329760879278183,
0.0538385733962059,
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0.05810893699526787,
0.0064262505620718,
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-0.029856711626052856,
0.013373577035963535,
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0.04388612136244774,
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<p>Sorry for this very technical question, but we suffer from this problem at work and I do not know any other place to ask. </p>
<p>Its about a good and cheap way of connecting a glass tube with a metal tube in a vacuum setup. There are some solutions with gradually changing glass-materials in order to minimize the tension caused by the different heating-extensions of metal and glass. However they are rather expensive and we would prefer a cheap solution. </p>
<p>We found a promising image of possible connection, however there was no manufacturer and no name of the flange given.
<img src="http://b1online.de/public/vacuum.jpg" alt="enter image description here"></p>
<p>So does anyone of you recognize this type of flange? Or does anyone know where to ask such a question? What would you suggest as a connection between metal and glass in a vacuum setup?</p> | g14377 | [
0.046270936727523804,
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0.01821071468293667,
0.005028851330280304,
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0.012882283888757229,
0.05683666840195656,
-0.06551385670900345,
0.0... |
<p>is there a theoretical limit to the amount of sound energy air can contain? In case, there is a limit, what is that limit? </p> | g14378 | [
0.03282758221030235,
0.10206131637096405,
0.022007068619132042,
0.01946953870356083,
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0.02262800745666027,
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-0.024020330980420113,
-0.014503199607133865,
0.004254307597875595,
-0.07245653122663498,
0.0... |
<p>I've always assumed that "quantum locking" was a term invented by the writers of Dr Who, but <a href="http://www.youtube.com/watch?v=Ws6AAhTw7RA" rel="nofollow">this video</a> suggests otherwise.</p>
<p>What is <em>quantum locking</em>? Is it real?</p> | g14379 | [
-0.03332306817173958,
0.1178450733423233,
0.0028120544739067554,
0.013624336570501328,
0.01342875137925148,
0.04056348279118538,
0.022105615586042404,
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0.04406890645623207,
-0.033451005816459656,
-0.041270818561315536,
0.016753308475017548,
-0.020885800942778587,
-0.0... |
<p>I am asked to calculate renormalization for a massless $\phi^3$ theory in $d=6$ dimensional space using dimensional regularization. I'm having trouble finding the three-point vertex correction as one has to choose correct renormalization conditions.
Does anyone know where I can read about this problem?</p> | g14380 | [
0.058598265051841736,
-0.00196707877330482,
0.004360738676041365,
-0.036852046847343445,
-0.03557805344462395,
0.024070393294095993,
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0.072618767619133,
0.012849193066358566,
0.037781961262226105,
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0.029327183961868286,
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0.0... |
<p>I began wondering about this when working through <a href="http://physics.stackexchange.com/questions/123525/why-should-a-physical-principle-be-applicable-to-different-systems-in-differen/123562?noredirect=1#comment251291_123562">this question</a>: is there anything analogous to spacetime or matter? Both spacetime (note: most of what I know of about the concept of spacetime comes from special and general Relativity) and mass are fundamental things in this universe. That is, our most fundamental physical laws contains measurements of position, time, and mass; they cannot be derived from anything. We know that space and time are inherently linked thanks to special and general relativity, but I can't think of anything else like spacetime. Is there even any other quantity that can be separated into dimensions?</p>
<p>Mass is similar in that we can't really get to a lower level of how much of something there is (volume is mostly empty space, not stuff, so it can vary when the amount of stuff remains constant). The thing that sets it apart from spacetime, though, is that space and time make up a coordinate system, in which everything is measured with respect to a defined origin (zero). Mass, however, is an absolute scale, so it seems to me that it is something fundamentally different.</p>
<p>Is there anything else like spacetime? Or mass? When thinking of fundamental quantities, I went to the base SI units. Besides position, mass, and time, they measure current (simplified to electric charge/time, and we've already discussed time, so let's just say electric charge here), thermodynamic temperature, luminous intensity, and number of moles (which is related to mass by a factor that's unique to each element, so we'll leave that as well and just use mass as the only measure for the amount of a substance).</p>
<p>So, electric charge, thermodynamic temperature, and luminous intensity seem to be the only logical choices for analogues. These are all manifested forms of energy (the quantifiable ability to cause change), and they are all absolute in scale. How does energy fit into all of this? (My instinct is to say it resembles mass, but then what is the "matter" of energy?)</p> | g14381 | [
0.04133765399456024,
-0.006895328871905804,
0.012288141995668411,
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0.0014820557553321123,
0.03942309319972992,
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0.0552898533642292,
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0.018274839967489243,
0.0... |
<p>I have a question about the right definition of the <a href="http://en.wikipedia.org/wiki/Green%27s_function" rel="nofollow">Green's function</a> in physics. Why do we introduce (or not) an infinitesimal, positive number $\eta$ to the following definition:</p>
<p>$$\left[ i\hbar\frac{\partial}{\partial t} - \hat{H}(\mathbf{r}) \pm i\eta\right]G(\mathbf{r},t;\mathbf{r'},t') = \delta(\mathbf{r} - \mathbf{r'})\delta(t-t')$$</p> | g14382 | [
0.016198663040995598,
0.04129756987094879,
-0.02051628939807415,
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0.055676545947790146,
0.005250090733170509,
0.0840158686041832,
0.03268889710307121,
-0.10154396295547485,
-0.048655446618795395,
-0.016840485855937004,
-0.020921021699905396,
0.061004187911748886,
0.007... |
<p><img src="http://i.stack.imgur.com/kWreF.gif" alt="enter image description here"></p>
<p>I don't understand how this could work. So lets say the drive is set to go 30,000,000 meters in a straight line to get to a distant planet. Now lets say the ship size is 20 meters. Now let's make the assumption that the warp field in the photo is 30 meters (10 meters bigger than the ship). So the warp bubble is applied. Now the 30 meters of space the ship occupied is cut. But it only could apply the warp bubble to this confined region which means 30 meters are cut instead of 30 million and now 29,999,970 meters are left and the ship is not even close to the planet it wants to go to (barely moves). Are there any solutions to this problem?</p> | g14383 | [
0.0034044254571199417,
0.02641809731721878,
-0.009235337376594543,
-0.05745219439268112,
0.02492653951048851,
0.05283406749367714,
0.008254894986748695,
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-0.07469040900468826,
0.03162765130400658,
0.020645618438720703,
0.017127754166722298,
-0.040... |
<p>this is a question I had when I was a kid. I'm a bit ashamed because I think I am missing out on something very obvious since I have the same question despite almost being an engineer now!</p>
<p>From Newtonian physics, I understand how although the gravitational force pulls the planet towards a star, and the planet 'falls' towards the star, due the angular momentum, it also moves laterally. In case of a planetary orbit, this is just enough to keep it moving around it in an elliptical orbit. (With speed and radius being such that angular momentum and energy is conserved)</p>
<p>But let's consider a single planet and star system where the planet moves around the star in a circular path for simplicity. If gravitational force provides centripetal force, what can account for the centrifugal force?</p>
<p>I have a feeling it is related to some inaccurate view I have, of centrifugal force. The <a href="http://www.xkcd.com/123/" rel="nofollow">xkcd comic</a> and <a href="https://en.wikipedia.org/wiki/Centrifugal_force" rel="nofollow">wikipedia article</a> talk about two concepts: fictitious and reactive centrifugal force. But it would be safe to assume the star(EDIT: sorry, I meant to choose a frame of reference such that the centripetal force on the planet has to be cancelled by a centrifugal force on the planet in the direction: star to planet) as a stationary frame of reference for the purpose of this question, right? Which means that it is not a fictitious force, right? Or is it not considered a centrifugal force if you take the star as the reference frame?</p> | g14384 | [
0.025699902325868607,
0.0158217903226614,
0.012726136483252048,
0.012945177964866161,
0.07890346646308899,
0.025362329557538033,
0.013612198643386364,
0.02999808080494404,
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-0.03971307352185249,
0.05279025807976723,
-0.005404762923717499,
0.02167973853647709,
-0.02800... |
<p>All books write $X(f)$ in plots - the Fourier transform of $x(t)$ - when they actually mean $\lvert X(f)\rvert$, without even mentioning in passing that they are dropping the mod sign. And also they consider only the mod of FT in the analysis, completely ignoring the phase term - especially communications analysis like AM and FM. Why? It is well known that in image processing phase also place an important role? Is there any proof or valid reason that phase is unimportant in communication engineering?</p> | g14385 | [
-0.030975090339779854,
-0.011936552822589874,
0.01816587522625923,
-0.04273844510316849,
0.03969154134392738,
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0.0003447073104325682,
0.016932161524891853,
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-0.04327242821455002,
0.0076009309850633144,
0.0008720578043721616,
0.027990616858005524,
0... |
<p>Is <a href="http://en.wikipedia.org/wiki/Velocity" rel="nofollow">velocity</a>:</p>
<ol>
<li><p>Distance/Time in a particular direction, or </p></li>
<li><p>simply Displacement/Time?</p></li>
</ol>
<p>Or both 1 and 2 is the same thing?</p> | g14386 | [
0.06393280625343323,
0.009842733852565289,
-0.021750248968601227,
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0.028234539553523064,
0.006621070671826601,
0.05297114700078964,
-0.0011280030012130737,
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-0.04473823681473732,
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0.01742912083864212,
0.042830850929021835,
-0... |
<p>Suppose that there is a wavefunction $\Psi (x,0)$ where 0 is referring to $t$. Let us also say that $a(k) = \frac{C\alpha}{\sqrt{\pi}}\exp(-\alpha^2k^2)$ is the spectral contents (spectral amplitudes) where $k$ is defined as wavenumber $k$. $\alpha$ and $C$ are constants. </p>
<p>My question is, why do we calculate $\Delta x$ by looking at where the value of $\Psi (x)$ diminish by $1/e$ from the maximum possible value of $\Psi (x)$? </p>
<p>Also, although the width of the $\Psi (x)$ packet is $4\alpha$, we define $\Delta x$ as $\alpha$. Why is it like this?</p>
<p>By the way, $\Delta x$ is used as in uncertainty principle.</p> | g14387 | [
0.029534809291362762,
0.010130365379154682,
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0.05174994841217995,
0.024879297241568565,
0.00911765918135643,
0.054172083735466,
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-0.010234108194708824,
-0.006230086553841829,
0.027591325342655182,
0.03719... |
<p>I am looking for a way to demonstrate that magnets adhere to the laws of thermodynamics, in particular the requirement that energy in a closed system be conserved.</p>
<p>To adhere to the requirement that energy not be lost, I would expect that the energy required to create a magnet would be offset by the energy exerted when that magnet exerts force.</p>
<p>My (elementary, if you will pardon the pun) understanding of magnets is that they exhibit a magnetic field because of a kind of polarization of the electrons, that looks something like this for a "perfectly" magnetized metal:</p>
<pre><code>---------------------
---------------------
---------------------
</code></pre>
<p>whereas an unmagnetized object would not have this polarization (if that is the right word), and might "look" something like this:</p>
<pre><code>|\/-/-\|-/|\/-/-\|-/
/|--/-\|-/|\/-|\\|\-
\-/-/-\-/|\/-|/-\|-\
</code></pre>
<p>The former object would exhibit the maximum force possible for the given material. The latter object would exhibit no magnetic force.</p>
<p>One would expect that the energy required to align the electrons would never exceed (but where inefficient it may be less than) the force that is exerted by the magnetic field.</p>
<p>As an example, suppose the polarization of a magnet from a completely unpolarized state uses 1 kJ of energy. To adhere to the laws of thermodynamics, the maximum amount of force the magnet can exert is 1 kJ, after which point it would be depolarized. One would expect the strength of the magnetic field to dissipate as it exerts force.</p>
<p>Is there a demonstration that one can perform to help visualize the conservation of energy by way of "charging" and "discharging" a magnet?</p> | g14388 | [
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0.0... |
<p>I am looking for references (books or web links) which have "simple" examples on the use of group theory in physics or science in general. </p>
<p>I have looked at many books on the subject unfortunately they usually require extensive technical coverage of the basics, i.e. the 1st 100 pages or something, to be able to start discussing applications.</p>
<p>I believe that there is an easy way to explain anything (it's just hard to find it).</p> | g14389 | [
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0.027257198467850685,
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0.021879499778151512,
0.07280870527029037,
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0.0... |
<p>I was going through a physics book which contains a short description of Nikola Tesla . The description mentioned about poly phase circuits (power supply). Out of curiosity I googled it up and I found that in a poly phase supply we use alternating currents of more than one frequency, say, w1, w2,w3,w4...</p>
<p>Can anyone tell me what was the logic behind the invention of polyphase power supply,or what forced Tesla to invent this polyphase power supply system?</p> | g14390 | [
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0.002166845602914691,
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0.013327470049262047,
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-0.02868138998746872,
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0.0... |
<p>Can anyone give a simple explanation for what the fractions and integers mean in particle physics when describing <a href="http://en.wikipedia.org/wiki/Spin_%28physics%29" rel="nofollow">spin</a>?
I've seen on another forum (the naked scientist) that it should not be thought of as angular momentum as described for spinning objects like the earth, so I don't understand what it actually is.</p>
<p>I also don't understand how it was measured from collisions i.e. if it was calculated from the momentum and energy of the collisions of particles?</p> | g14391 | [
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-0.026317691430449486,
-0.02627289853990078,
-0.0012427727924659848,
0.05975504219532013,
-0.0... |
<p>I have seen this question:
<a href="http://physics.stackexchange.com/questions/6483/why-can-you-see-virtual-images">Why can you see virtual images?</a> but answers evade the question. Light needs to hit the retina in order, what is the meaning of "dashed" lines in most visualizations that track the light back near the original object? Can someone clarify? Is it reflected?</p> | g14392 | [
-0.01647971011698246,
0.05002262815833092,
-0.019355248659849167,
0.016162076964974403,
0.06639162451028824,
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0.038694433867931366,
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-0.045665353536605835,
0.004112971015274525,
0.05475826561450958,
0.08518238365650177,
0.0330... |
<p>I understand that general relativity is applicable to gravitational fields and special relativity is applicable to case when there is no gravity. But is there a derivation on how to reduce General Relativity to Special Relativity in limiting case, much like how General Relativity is reduced to Newtonian gravity in weak-gravity case?</p>
<p><strong>Edit: By <em>reducing</em> I mean, how can we derive the Lorentz transformation from General Relativity under appropriate limits?</strong></p> | g14393 | [
0.01681312546133995,
0.051684603095054626,
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0.035482700914144516,
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0.019614415243268013,
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0.042840566486120224,
0.006... |
<p>In Einstein's papers, he used light speed as a reference speed. What if we use a greater finite speed and do the same calculations. Won't this greater speed then be the limit.</p> | g14394 | [
0.008008572272956371,
0.08187682181596756,
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0.013262801803648472,
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0.0023224635515362024,
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-0.04462045803666115,
0.05389540269970894,
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0.0415... |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/10612/explain-how-or-if-a-box-full-of-photons-would-weigh-more-due-to-massless-photo">Explain how (or if) a box full of photons would weigh more due to massless photons</a> </p>
</blockquote>
<p>It has been proved and showed through experiments that light can be bent by the Sun or any other body with considerable mass. Also light is nothing but photons. So can these photons be attracted by massive bodies if they have no mass?</p> | g143 | [
0.0038306687492877245,
0.022647125646471977,
0.00524362875148654,
0.009163560345768929,
0.005376564804464579,
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0.042813003063201904,
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-0.041044559329748154,
0.012411870062351227,
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0.021318446844816208,
0... |
<p>In a graph on reflection, can i give the x axis a scale of 4 cms = 10 degrees , and the y-axis a scale of 2 cms = 10 degrees? Will it affect the readings? </p> | g14395 | [
0.042316265404224396,
0.04402271285653114,
0.007564104627817869,
-0.06896005570888519,
0.039749257266521454,
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-0.0049986932426691055,
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... |
<p>In 3D, I can calculate the total force due to gravity acting on a point on the surface of the unit sphere of constant density, where I choose units so that all physical constants (as well as the density of the sphere) is 1:</p>
<p>$$F = 4\int_{-1}^1 \int_{-\sqrt{1-x^2}}^{\sqrt{1-x^2}} \int_{-\sqrt{1-x^2-y^2}}^{\sqrt{1-x^2-y^2}} \tfrac{x+1}{\left[(x+1)^2+y^2+z^2\right]^{3/2}} dz\, dy\, dx = \frac{16\pi}{3}.$$</p>
<p>This force agrees with what we get if we treat the sphere as a point particle at the sphere's center of gravity with lumped mass $\frac{4\pi}{3}$. So far so good.</p>
<p>But now if I try to calculate the force of gravity of a point on the boundary of the unit disk in 2D, I get infinity:</p>
<p>$$F = 2\int_{-1}^1 \int_{-\sqrt{1-x^2}}^{\sqrt{1-x^2}} \tfrac{x+1}{\left[(x+1)^2+y^2\right]^{3/2}} dy\, dx = \infty?$$
Intuitively, if I think of the total force as the sum of contributions from nested circular rings around the point of interest, the circumference of the rings scales like $r$, the force density like $\frac{1}{r^2}$, so the contribution of each ring scales like $\frac{1}{r}$ which diverges as $r\to 0$.</p>
<p>What is going on here? How is the gravity potential derived? Is it an accident that in 3D, $\frac{G m_1 m_2}{r}$ is harmonic on the punctured space $\mathbb{R}^3 \backslash\{0\}$? Is the "right" gravity potential in 2D something like $G m_1 m_2 \log r$? If so, why, and isn't it a paradox if point masses in 2D orbit according to a different law than coplanar point masses in 3D?</p> | g14396 | [
0.02162812277674675,
0.02456667460501194,
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0.05867591127753258,
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0.06417728215456009,
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-0.00019729587074834853,
0.003491937415674329,
0.007268096785992384,
0.020275378599762917,
-0... |
<p>Let us assume that there exists a Hamiltonian that (together with the initial state) describes the whole universe.</p>
<p>Then my question is :
What is the spectrum of this Hamiltonian and what are the multiplicities of the spectrum ?
More precisely, what is the representation of the Hamiltonian in Spectral multiplicity theory (see <a href="http://en.wikipedia.org/wiki/Self-adjoint_operator" rel="nofollow">http://en.wikipedia.org/wiki/Self-adjoint_operator</a>, section "Spectral multiplicity theory").</p> | g14397 | [
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0.025251485407352448,
-0.04290591925382614,
0.019082125276327133,
0.007644730154424906,
0.078682... |
<p>I am currently wondering about this famous rule:</p>
<p><img src="http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Daniel_Lenord/righthand.jpg" alt="picture"></p>
<p>Where does it come from mathematically that when you point with your thumb in the direction of the current, your curved fingers will point in the direction of the $B$-field? In other words, which mathematical operation does it come from? Probably it has to do with some vector product or Stokes theorem application, but I am not quite sure about it.</p> | g14398 | [
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0.057581137865781784,
0.008628351613879204,
... |
<p>While searching in web regarding whether rate of change of acceleration is possible or not; I came across the concept of <a href="http://en.wikipedia.org/wiki/Jerk_%28physics%29" rel="nofollow">jerk</a>. I want to know whether photons which can be accelerated can also have jerk or not?</p> | g159 | [
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0.014... |
<p>I expand metric $g_{ij}$ about flat space $\delta_{ij}$</p>
<p>$$g_{ij}=\delta_{ij}+h_{ij}$$ </p>
<p>where $|h_{ij}|\ll 1$.</p>
<p>I would like to find $R_{ij}$, to linear order, in terms of $h_{ij}$, but I dont know what terms are negligible. Which $\partial h \partial h$, $\partial h$, $hh, h \partial h $, $\partial \partial h$ are negligible?</p> | g14399 | [
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... |
<p>I have thought over this problem but I haven't found the solution: There is an electric charge $q$ at a distance $d$ from a conducting slab with thickness $t$, the problem is to find the potential everywhere, assumming that the slab is grounded. So, on the side of the slab where the charge is I think one may use the image method solution to find the potential in this region of space. Inside the slab $\phi =0$, but the question remains how to find the potential on the other side? The method of images does not seem to work on that side(?), and other solutions seem difficult to implement. Charge conservation seems to imply that the total charge induced on each surface of the slab is $\pm q$.
Is there any easy or semi-easy way to do it? I guess my question would be: what is the most straightforward way to do it?
Any help is greatly appreciated.</p> | g14400 | [
0.06547088176012039,
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0.011856620199978352,
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0.05575298145413399,
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0.05045550689101219,
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0.0548... |
<p>I heard a few times that using them as sunglasses is hurting the eye since UV light is not filtered, but the pupil is wider than it would be w/o wearing them because the visible light is dimmed.</p>
<p>I have always assumed since atmospheric (i.e. not reflected from flat surfaces) sunlight is unpolarised and since the glasses used in 3D cinema only allow circular polarised light to pass they do indeed filter out a large portion of the intensity of the incoming (unpolarised) light. </p>
<p>Does this not work for the UV part of the spectrum or is my initial assumption wrong?</p> | g14401 | [
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<p>Due to a previous <a href="http://physics.stackexchange.com/q/76158/6316">question</a>, I am confused with the expectation value of the stress-energy tensor in a 2-D conformal field theory.</p>
<p>Let's take the example of string theory, to sketch the problem.
Defining the action by $S = \frac{1}{2 \pi \alpha'}\int d^2z \partial X^\mu \bar \partial X_\mu$, the (euclidean) expectation value of an operator is : </p>
<p>$$\langle\mathcal F[X]\rangle=\int [dX]~ exp(-S)~F[X]\tag{0}$$</p>
<p>My questions are : </p>
<p>1) In string theory, what is :
$$\langle \partial X^\mu \partial X_\mu\rangle \tag{1}$$
2) In string theory, what is :
$$\langle :\partial X^\mu \partial X_\mu:\rangle \tag{2}$$
3) In a general 2-D conformal field theory, what is :
$$\langle T^{\alpha \beta}\rangle \tag{3}$$</p> | g14402 | [
0.02229922264814377,
0.0014661074383184314,
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0.00742914155125618,
0.05080209672451019,
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0.028561316430568695,
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-0.... |
<p>I am struggling with a problem from classical mechanics. Imagine a massless wheel (to make it simpler) with a mass $m$ fixed to it rolling without slipping on a horizontal ground. If we now try to find the equations of motion of the wheel (for instance the angle $\alpha$ it turns) we will find that all the forces are independent of velocity, so $\alpha ''=f(\alpha)$.</p>
<p>After doing that I decided to solve this problem using Euler-Langrange equations (since friction does no work). I came up with $L=\frac{1}{2} m R^{2} (\frac{d\alpha}{dt})^{2} (1+\cos \alpha)-mgR(1+\cos \alpha)$ which, upon solving, gives $\alpha ''$ as a function of both $\alpha$ and $\alpha '$. What is my problem?</p> | g14403 | [
0.011967799626290798,
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0.020243294537067413,
0.050269681960344315,
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... |
<p>What practical application can we expect from particle physics a century or two from now? What use can we make of quark-gluon plasmas or strange quarks? How can we harness W- and Z-bosons or the Higgs boson? Nuclear physics has given us nuclear plants and the promise of fusion power in the near future. What about particle physics? If we extend our timeframe, what promise does string theory give us? Can we make use of black holes?</p> | g14404 | [
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<p>This question <a href="http://physics.stackexchange.com/questions/7421/brachistochrone-problem-for-inhomogeneous-potential">Brachistochrone Problem for Inhomogeneous Potential</a> has the obvious extension. Namely the same question, when gravity is treated according to general relativity. To make it specific let's consider the case of Schwarzschild metric. Given to points, outside the event horizon, what is the trajectory (or the worldline) of a test body, which goes from the first point to the second for least time. All considered from the point of view of a distant observer at rest (with respect to the star/back hole). Of course other metrics or the general case are also interesting. </p>
<p>I couldn't find much by searching on the net. There is one article with the tittle suggesting relevance (actually more but this one seems very relevant), but it is over 60 pages of technicalities. So it would be nice to see a shorter answer.</p> | g14405 | [
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0.02454882115125656,
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0.02104186825454235,
0.001650607562623918,
-0.003235... |
<p>In the Cohen-Tannoudji Quantum Physics book, Complement BII, says:</p>
<p>[...] two operators $A$ and $B$ with both commute with their commutator. An argument modeled on the preceding one shows that, if we have:</p>
<p>\begin{align}
[A,C]=[B,C]=0
\end{align}
with $C=[A,B]$, then:
\begin{align}
[A,F(B)]=[A,B]F'(B)
\end{align}</p>
<p>Then this last property, is used to proof Glauber's Formula.
\begin{align*}
e^Ae^B=e^{A+B}e^{\frac{1}{2}[A,B]}
\end{align*}</p>
<p>I understand this proof, but I couldn't find a way to demonstrate,</p>
<p>\begin{align}
[A,C]=[B,C]=0
\end{align}
with $C=[A,B]$, then:
\begin{align}
[A,F(B)]=[A,B]F'(B)
\end{align}</p>
<p>I would like to know to do this, so I can understand better the Glauber's Formula proof.</p> | g14406 | [
0.02275056019425392,
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0.0007677820976823568,
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0.07612306624650955,
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0.0500919334590435,
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0.020365800708532333,
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0.021... |
<p>I'm a computer programmer that never studied physics in school and now it's coming back to bite me a bit in some of the stuff I'm being asked to program. I'm trying to self study some physics and I've got a few open source intro physics books and understanding it for the most part but I'm a bit confused on this statement I've stumbled upon in a section about static friction.</p>
<p>It's already gone over the formula for static friction and so forth. It gets into a section explaining that the weight of a train increases static friction between the wheels and the tracks. Alright, makes sense. But then it says this:</p>
<blockquote>
The reason locomotives are built to be so heavy is for traction.
The upward normal force of the rails on the wheels, FN, cancels
the downward force of gravity, FW, so ignoring plus and minus
signs, these two forces are equal in absolute value, FN = FW.
Given this amount of normal force, the maximum force of static
friction is Fs = sFN = sFW. This static frictional force, of the
rails pushing forward on the wheels, is the only force that can
accelerate the train, pull it uphill, or cancel out the force of air
resistance while cruising at constant speed. The coefficient of
static friction for steel on steel is about 1/4, so no locomotive can
pull with a force greater than about 1/4 of its own weight. If the
engine is capable of supplying more than that amount of force, the
result will be simply to break static friction and spin the wheels.
<br /><br />
- "Newtonian Physics", Light and Matter - Book 1, p158 B. Crowell
<a href="http://www.lightandmatter.com/bk1.pdf">http://www.lightandmatter.com/bk1.pdf</a>
</blockquote>
<p>I'm confused as to how static friction is the only thing that can move the train forward. I thought static friction was what kept it in place in the first place. There's another force - that I can't think of the name of, but I've heard of somewhere - that I thought was more what they're describing here, where the weight of the wheels pushing down and forward slightly on the tracks causes the tracks to push up and forward (from the opposite side).</p>
<p>Can someone explain to me what this is saying?</p> | g14407 | [
0.04362647235393524,
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0.04054272174835205,
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0.07864859700202942,
0.008722133003175259,
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0.009043942205607891,
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0.040094368159770966,
-0.03320264... |
<p>We know, the rockets in space use Newton's 3rd law to increase their velocity and hence move. What I don't understand is how it is possible in space aka vacuum-state without air? From what I know, Joule's "Free Expansion of Gas" says that free-expansion compresses the gas and is therefore "affected" by vacuum so it can't make the rocket move as the gas will have zero press/force. Could someone please explain me how rockets do really work and the above-mentioned statement? </p>
<p>Actually, please have a look at this site: <a href="http://cluesforum.info/viewtopic.php?f=23&t=1632" rel="nofollow">http://cluesforum.info/viewtopic.php?f=23&t=1632</a></p>
<p>Not: The site appears to include some conspiracy theory thingummies, but made me wonder anyway.</p> | g14408 | [
0.010027525946497917,
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-0.... |
<p>Earth moves with a speed around $100000 \,\,km/h$, which will be amazing, but from where we can see that moving speed? If we far away from earth (outside of earth gravity) then can view its moving? is any video already taken? (I failed to search that kind of video).</p> | g14409 | [
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0.07659237086772919,
0.03833374008536339,
0.06125738471746445,
0.062218613... |
<p>Is it possible to demonstrate that there are just 14 types of Bravais lattice without the knowledge of group theory?</p> | g14410 | [
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0... |
<p>The "equal angles" law of refection on a flat mirror is a macroscopic phenomenon. To put it in anthropomorphic terms, how do individual photons know the orientation of the mirror so as to bounce off in the correct direction?</p> | g92 | [
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<p>If something is moving away fast it's color would appear redder than its actual color due to phenomenon known as Doppler effect. </p>
<p>But what if the object isn't moving but i am moving away faster would that still affect the way waves travel. </p>
<p>Optional:-</p>
<p>And why. Object emitting isn't moving. And won't that means that doppler effect is subjective. What does that say about light and its wavelength?</p> | g14411 | [
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0.04853319749236107,
0.063645... |
<p>We have discovered quite a number of exoplanets to date. The Kepler spacecraft has examined 150,000 stars and found <a href="http://exoplanet.eu/catalog/">1,059 exoplanets</a>.</p>
<p>We know that Kepler, as well as all other exoplanet searches to date, can only find planets that cross in front of their star. That means that a large percentage of existing planets will not be detected. In addition, a lot of planets may be too small for the sensitivity of our detectors.</p>
<p>From these facts, has anyone calculated the probability of a star having a planetary system? I am guessing it is likely quite close to 1, i.e. all stars have planets, but I have not seen anyone figures for it. </p> | g14412 | [
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<p>In Landau-Lifschitz, following expansion is given,
We have,
$$L(v'^2)~=~L(v^2+2\textbf{v}\cdot\epsilon+\epsilon ^2)$$
expanding this in powers of $\epsilon$ and neglecting powers of higher order,
$$L(v'^2)~\approx~L(v^2)+\frac{\partial L}{\partial (v^2)}2\textbf{v}\cdot\epsilon$$</p>
<p>$L$ is Lagrangian. and $\textbf{v}'=\textbf{v}+\epsilon$.
I am unable to follow what expansion this exactly is. It does not look like Taylor expansion.</p> | g962 | [
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<p>How can I derive the following formula?
$$\int d^{d+1} k \frac{e^{i K X}}{K^2} = \frac{\Gamma (d-1)}{(4\pi)^{d/2} \Gamma (d/2) |X|^{d-1}}, \quad K^2 = k_0^2 + \vec k^2, KX = k_0 \tau + \vec k \vec x$$
What I tried so far:</p>
<ol>
<li><p>Integrate over $k_0$ while promoting $k_0$ to complex variable, so I can use residue theorem. Then I move to <a href="http://en.wikipedia.org/wiki/N-sphere#Spherical_coordinates" rel="nofollow">n-spherical coordinates</a>. I obtain:
$$ \propto {\rm sign} (\tau) \int d \Omega \int dr \, r^{d-3} e^{-r |\tau| + ir |\vec x| \cos \phi_1}$$
And I'm stuck.</p></li>
<li><p>I change variables to n-spherical ones and I end up with this:
$$\int d \Omega \int dr \; r^{d-2} e^{ir |\vec x| \cos \phi_1}$$
And I'm stuck as well.</p></li>
</ol> | g14413 | [
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0.034656770527362823,
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0.0... |
<p>In physics entropy is usually measured in <a href="http://en.wikipedia.org/wiki/Nat_%28information%29" rel="nofollow">nats</a>. I wonder is there a possible model of a physical system which has entropy of discrete number of nats?</p>
<p>How particles and degrees of freedom should be arranged so this to happen?</p>
<p>I would be interested in both </p>
<ul>
<li><p>example of an analog (non-discrete) system</p></li>
<li><p>example of a quantum system</p></li>
</ul> | g14414 | [
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<p>I have recently been intrigued by the following question: What is the difference between the pitch of the noise of dripping water between hot and cold water? For example, would cold water create a higher pitched noise while dripping into a pot of water? Or vice versa?</p> | g14415 | [
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0.04440154880285263,
0.026... |
<p>In the <a href="http://web.njit.edu/~kappraff/chapter19.pdf" rel="nofollow">this</a> chapter of an online pdf we are given an equation for the deflection of a beam:
$$\frac{d^2y}{dx^2}=\frac{\overline{M}}{E I}$$
where $E$ is the modulus of elasticity, $\overline{M}$ is the moment as function of $x$ and $I$ is the moment of inertia. It works nicely for a beam with one free end as the beam would be rotating around a single axis. However when they get to the worked example with both ends fixed they leave the moment of inertia uncalulated (left just as $I$). I am wondering how one would calculate the moment of inertia for such a beam. Is it even defined?</p> | g14416 | [
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0.03272666782140732,
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... |
<p>Let's start with kinetic energy (from <a href="http://en.wikipedia.org/wiki/Kinetic_energy">los Wikipedias</a>)</p>
<blockquote>
<p><em>The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body in decelerating from its current speed to a state of rest. In classical mechanics, the kinetic energy of a non-rotating object of mass m traveling at a speed $v$ is $\frac{1}{2}mv^2$.</em></p>
</blockquote>
<p>Let's say you & your bike have mass of 100kgs, then your kinetic energy at 10m/s would be</p>
<p>$$ E_a = 1/2 \times 100 \times 10^2 = 5000J = 5kJ$$</p>
<p>If you apply another 5kJ of energy, you don't get to 20m/s though, you only get to:</p>
<p>$$ E_b = 10000J =1/2 \times 100 \times V_b^2$$
$$\implies V_b = \sqrt{10000 / (1/2 \times 100)}
= √200
= 14.14m/s$$</p>
<p>Let's say you and a buddy are both coasting along at 10m/s though, from their perspective you've just burned 5kJ but only accelerated 4.1m/s, <em>even though you seemed stationary</em>.</p>
<p>Imagine you and your mate are in space drifting along together, at an unknown speed. Your mate fires his burners and accelerates away from you. There's a big screen on his ship showing how many joules of energy he just burned, and you can measure his resulting relative velocity just fine.</p>
<p>The question is, Will 5kJ of energy always produce 10m/s of relative velocity,,assuming 100kg spaceships?</p>
<p>If 5kJ always produces 10m/s, Why does the second 5kJ only produce 4.1m/s? What is going on here?</p> | g14417 | [
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-0.0038... |
<p>Photons of relic radiation loose their energy as they propagate through space. Will a space traveler loose their peculiar velocity as he travels through vast distances? Will he stop somewhere or still he would be able to reach the stars at unlimited distance without burning the engines?</p> | g14418 | [
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-0.0345... |
<p>Saw a question about faster than light travel... I still have the same question though none of the answers offered any resolution for me.
It is so summarily assumed by all physicists and commentaries that exceeding the speed of light would turn back the clock. I can't see the relation. </p>
<p>Doubling the amount of any speed halves the time taken to travel a given distance. Keep doubling the speed and that time is halved (or otherwise divided). Divide any quantity (time in this case) and you always end up with a fraction of it but never zero and certainly never a negative amount as would be the case for the causality conflict.
So it seems to me that whatever speed one attains, there is always a positive time element in the travel no matter how tiny!!</p>
<p>The speed of light is only unique to me in that it is the fastest observed speed but is otherwise just another speed quantity set by nature (just like the speed of sound etc) could it be that other elements in nature are travelling faster than light but we lack the means to detect or measure them (like the rebellious neutrino)?</p>
<p>I also don't understand time as an independent element that can be slowed sped up etc.
It seems to me that time is simply a relative measure of the ever-changing state of matter relative to other states of matter.</p>
<p>If every thing in the universe stopped- that is all state of matter everywhere frozen, all electrons frozen in place etc wouldn't we observe that time had stopped? Isn't it therefore our observation of the changing state of matter around us that gives the perception (perhaps illusion) of time? </p>
<p>I can therefore only understand time as a subjective sense of changing states relative to an observer! It should be the rate of change of these states that slow down or speed up (in relation to the observer or instrument) and not the universal rate of change or universal time that changes! It would also debunk any notion of time travel, as it would involve the manipulation of every particle in the universe to a previous of future state...</p>
<p>Disclaimer.. I hate calculations, stink at them and have no idea what mathematical formulas are used to arrive at the accepted conclusions so I'm not trying to dispute any findings etc by the experts, just trying to align my lay understanding to their conclusions.</p> | g14419 | [
0.05730857700109482,
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0.04709552228450775,
0.008797... |
<p>There are no book recommendations for Astrophysics here. I will write my own answer, but I am also interested in what are others' views on the question (I will NOT mark my own answer as the best one).</p> | g741 | [
0.03552408516407013,
0.05271089822053909,
0.02049090340733528,
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0.011666682548820972,
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-0.006266400218009949,
0.034027375280857086,
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0.011665701866149902,
-0.... |
<p>Reading <a href="http://physics.stackexchange.com/questions/1019/common-false-beliefs-in-physics">Common false beliefs in Physics</a> I feel that there are also "fake false beliefs", ie, that the "false belief" does not actually exists, it is a rhetorical argument or just a basis for argumentation, but not actually a belief... while the meta-belief on its existence actually exists.</p>
<p>Prototypical example is mid-age "flat-earth". All the discussion with Colon was about the diameter of the earth, not about the flatness or sphericity. And still, you can hear here and there that the mid-age philosophers had the "false belief" of a flat earth.</p>
<p>A more dubious example: does people actually think that summer is related to the distance to the sun? Or is it a rhetorical trick of the teacher, to induce this thinking during the classroom only to correct it later? Most countrymen clearly perceive that summer is about height of the sun, not distance.</p> | g14420 | [
0.05818679556250572,
0.028013266623020172,
0.004561710171401501,
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0.03258337825536728,
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0.004494106862694025,
0.05... |
<p>Could you please recommend a sufficiently elementary introduction to K3 gravitational instanton in general relativity and the problem of finding its explicit form?</p>
<p>Under 'sufficiently elementary' I mean the texts geared at physicists with basic knowledge of general realtivity but not much more than that (i.e. with almost no knowledge of modern differential geometry and algebraic geometry). To give a hint, I find the description of the problem in the book <em>Solitons, Instantons and Twistors</em> by Maciej Dunajski way too terse.</p>
<p>Many thanks in advance!</p> | g14421 | [
-0.00003643946547526866,
0.021752385422587395,
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0.052211809903383255,
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0.05734998360276222,
... |
<p>In case of the gauge-fixed Faddeev-Popov Lagrangian:
$$ \mathcal{L}=-\frac{1}{4}F_{\mu\nu}\,^{a}F^{\mu\nu a}+\bar{\psi}\left(i\gamma^{\mu}D_{\mu}-m\right)\psi-\frac{\xi}{2}B^{a}B^{a}+B^{a}\partial^{\mu}A_{\mu}\,^{a}+\bar{c}^{a}\left(-\partial^{\mu}D_{\mu}\,^{ac}\right)c^{c} $$</p>
<p>(for example in Peskin and Schröder equation 16.44)</p>
<p>If you expand the last term (for the ghost fields) you get:
$$ \bar{c}^{a}\left(-\partial^{\mu}D_{\mu}\,^{ac}\right)c^{c} = -\bar{c}^{a}\partial^{2}c^{a}-gf^{abc}\bar{c}^{a}\left(\partial^{\mu}A_{\mu}\,^{b}\right)c^{c}-gf^{abc}\bar{c}^{a}A_{\mu}\,^{b}\partial^{\mu}c^{c} $$</p>
<p>And so, the Lagrangian has a term proportional to the second derivative of $c^a$.</p>
<p>In this case, how does one find the classical equations of motion for the various ghost fields and their adjoints?</p>
<p>I found the following equations of motion so far:
$$ D_{\beta}\,^{dc}F^{\beta\sigma}\,^{c}=-g\bar{\psi}\gamma^{\sigma}t^{d}\psi+\partial^{\sigma}B^{d}+gf^{dac}\left(\partial^{\sigma}\bar{c}^{a}\right)c^{c} = 0 $$
$$ \sum_{j}\partial_{\sigma}\bar{\psi}_{\alpha,\, j}i\gamma^{\sigma}\,_{ji}-\sum_{\beta}\sum_{j}\bar{\psi}_{\beta,\, j}\left(gA_{\mu}\,^{a}\gamma^{\mu}\,_{ji}t^{a}\,_{\beta\alpha}-m\delta_{ji}\delta_{\beta\alpha}\right)=0 $$
$$ \left(i\gamma^{\mu}D_{\mu}-m\right)\psi=0 $$
$$ B^{b}=\frac{1}{\xi}\partial^{\mu}A_{\mu}\,^{b} $$
$$ \partial^{\mu}\left(D_{\mu}\,^{dc}c^{c}\right)=0 $$
$$ f^{abd}\left(\partial_{\sigma}\bar{c}^{a}\right)A^{\sigma}\,^{b}=0 $$</p>
<p>But it is the last equation that I suspect is false (I saw the equation $ D_\mu\,^{ad} \partial^\mu \bar{c}^d = 0 $ in some exercise sheet (<a href="http://www.itp.phys.ethz.ch/education/fs14/qftII/Series7-3.pdf" rel="nofollow">http://www.itp.phys.ethz.ch/education/fs14/qftII/Series7-3.pdf</a> Exercise 3) and I also saw the equation $D^\mu\,^{ad}\partial_\mu B^d = igf^{dbc}(\partial^\mu\bar{c}^b)D_\mu\,^{dc} c^c$ which I don't understand how they were derived.)</p>
<p>EDIT: Thanks to Qmechanic's answer I was able to derive the correct equations of motions (as noted in the comment to that answer) but I still don't know where to "obtain" the last equation I mentioned which connects the auxiliary field with the ghost fields.</p> | g14422 | [
0.053658537566661835,
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0.05678201839327812,
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0.030608588829636574,
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0... |
<p>I am trying to reproduce the number of participants $N_{part}$ in Pb-Pb Collisions at LHC using a Glauber Monte Carlo simulation, specifically aiming to reproduce the values given in <a href="http://prl.aps.org/abstract/PRL/v106/i3/e032301" rel="nofollow">Phys. Rev. Lett. 106, 032301 (2011)</a> by K. Aamodt et al. (the ALICE Collaboration). To do this, I am looking for the inelastic nucleon-nucleon cross section.</p>
<p>A paper on the software written for the PHOBOS experiment (<a href="http://arxiv.org/abs/0805.4411" rel="nofollow">available on arXiv here</A>) suggest that it is (at LHC energies) "<em>expected to be around $\sigma_{NN} = 72$ mb (with large uncertainty from the unknown elastic cross section)</em>".</p>
<p>I have been using <a href="http://www-linux.gsi.de/~misko/overlap/" rel="nofollow">this web-interface provided by the GSI</a> to get an estimate of $N_{part}$. The website suggests to use $60$mb at $\sqrt{s_{NN}} = 5500 \text{GeV}$. However, even using the 72mb suggested above, for 0%-5% centrality I only get about $376$ participant nucleons, compared with $N_{part} = 382.8$ for $2.76$ TeV published in the ALICE paper. </p>
<p>I looked at the <a href="http://pdg.lbl.gov/2011/hadronic-xsections/hadron.html" rel="nofollow">PDG website</a> as well, but since (as hinted above) the plots do not show data for the elastic part of the cross-section, no value is provided for the energy I'm looking at. The fit funciton provided does not seem to give a reasonable value for LHC energies.</p>
<blockquote>
<p>Am I right in assuming that we simply don't know the value of $\sigma_{NN}$ yet and that it must thus be estimated for such calculations?
If that is the case, is there a common way to do so? </p>
</blockquote> | g14423 | [
0.025215713307261467,
0.023208821192383766,
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... |
<p>In a rectangular room 10*8*5 how can the eigen modes belonging to octave and third octave bands centered at 50Hz be found ? I have the formula to calculate the number of modes but don't have the idea how to find the modes. Do I need to use Matlab for it ?</p> | g14424 | [
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0.05... |
<p>This <a href="http://www.physicsclassroom.com/class/estatics/Lesson-4/Electric-Fields-and-Conductors" rel="nofollow">site</a> says that if the field at the surface at the conductor has a parallel component, then the surface charge will move, which is impossible if the conductor is at equilibrium. </p>
<p>But I learnt that positive ions can't move in a conductor, only electrons can.</p>
<p>I asked my prof and he said that if the field has a parallel component, the parallel components will cause the electrons to move, which will result in movement of positive charge.
<img src="http://i.stack.imgur.com/DocHi.png" alt="enter image description here"></p>
<p>1) But an electric field is a vector field, so each point has a different vector associated with it. So what would happen if the vector field had parallel components where positive ions are located but is completely perpendicular where electrons are located (look at the above image)? Isn't this a scenario where the entire field isn't perpendicular but there is no movement?</p>
<p>2) If a perpendicular field is acting on the electron, what's stopping it from leaving the conductor? Normally, if an electron tries to leave because of a field, the positive charges redistribute themselves to cancel out the field, preventing it from leaving. But if the field isn't cancelled out, theres nothing holding the electron back is there?</p> | g14425 | [
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0.007140153553336859,
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0.04632127657532692,
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-0.... |
<p>An electric lamp having coil of negligible inductance connected in series with a
capacitor and an AC source is glowing with certain brightness. How does the
brightness of the lamp change on reducing the (i) capacitance, and (ii) the frequency ? What if the diaelectric slab is introduced between the capacitor.</p> | g14426 | [
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<p>Is the Virgo Supercluster affected at all by the expansion of space? What about the larger filament that Virgo resides in?I don't quite understand how gravity affects space time locally in groups of galaxies, and am wondering if the expansion of space is lumpy. It would be nice to know we are not all being stretched... :)</p> | g14427 | [
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<p>Let's say there is a charged tube(cylinder with no top or bottom) with radius $a$, length $l$ and charge $q$ and a point which is collinear with the centre of the charged tube. Anyway, since we can think the tube as infinite charged rings we first should calculate the electric field of a single ring.(the distance between ring and point charge is $r$ and the distance between the centre of the ring and point charge is $d$)
$$
dE= (k.dq)/r^2
$$
where $dq$ is infinitesimal charge.
$$
dE_x=(k.dq)/r^2cos(\theta)
$$
$$
dE_x=(k.d.dq)/(a^2+d^2)^{(3⁄2)}
$$
$$
E_x=∫(k.d.dq)/(a^2+d^2)^{(3⁄2)} =(k.d)/(a^2+d^2)^{(3⁄2)}∫dq=kdq/(a^2+d^2)^{(3⁄2)}
$$
Finally, we found the electric field of a single ring. However, I am stuck at how to integrate the equation to find the total electric field of the charged tube. How should I integrate it?</p>
<p>P.S: It may seem like the <a href="http://physics.stackexchange.com/questions/131831/how-to-calculate-the-electrostatic-force-between-two-dielectric-rings-of-charge">other question</a> I asked but they are different in concept of what the question is.</p> | g14428 | [
0.038422029465436935,
0.023816434666514397,
-0.026978058740496635,
-0.015428411774337292,
0.08024556189775467,
0.021740825846791267,
0.05190642923116684,
0.0339941680431366,
-0.06232630833983421,
0.025265123695135117,
-0.06444007158279419,
-0.008057858794927597,
-0.013667687773704529,
-0.0... |
<p>Our measure of distance (the meter) is defined in terms of how far light in a vacuum travels in a specific time.</p>
<p>When light travels through another medium, we say it travels at a different speed. Why not say it travels at the same speed, and the distance in the medium is different that the distance outside? Is it only because the 2nd explanation feels less intuitive than the first, or is there more to it? </p> | g14429 | [
0.011298418045043945,
-0.004042519256472588,
0.002785396995022893,
0.029305865988135338,
0.031247548758983612,
0.02562333643436432,
0.004237377550452948,
-0.0029862027149647474,
-0.04803464561700821,
-0.027568278834223747,
0.03015289083123207,
-0.01724807731807232,
0.05120306834578514,
0.0... |
<p>When doing calculations in rigid body mechanics, it is necessary to choose an origin to calculate torques and angular momenta. However, the underlying dynamics does not depend upon the choice of that origin. This is very reminiscent of gauge theories. Could we interpret the choice of origins as a gauge fixing? Has gauge formalism been applied to rigid body mechanics?</p> | g14430 | [
0.06019860878586769,
-0.048242729157209396,
-0.008892028592526913,
0.023248713463544846,
0.057311926037073135,
-0.01541387103497982,
0.05792078748345375,
0.04832008481025696,
-0.0015675821341574192,
0.025387605652213097,
0.03251245245337486,
-0.04650669917464256,
0.04327216371893883,
-0.00... |
<p>I've already tried to find the expression but I don't find the expected result for the quantity of charge when I use the expression I found. For example, I checked it by integrating the expression that I found from $r=0$ to $r=\frac{L}{2}$, it should lead to $Q=\frac{\lambda L}{2}$ but it is not what I find.</p> | g14431 | [
0.06498586386442184,
0.006717560347169638,
-0.0104479705914855,
-0.03226642310619354,
0.07061304897069931,
-0.02246173657476902,
0.0153832221403718,
0.0024259150959551334,
-0.06524231284856796,
-0.05405838415026665,
-0.05596855282783508,
0.024362225085496902,
0.009118464775383472,
0.042012... |
<p>The capacitor has insulator and it is covered by two conductors from top and bottom.
I started to think if this insulation part can be wide like 1 cm big - yes, it can but then permittivity has to be great for great capacitance. </p>
<p>If the insulator is air, then I think the capacitor is very weak.
Capacitance is</p>
<p>\begin{equation}
c = \epsilon/d
\end{equation}</p>
<p>where $c$ is capacitance, $\epsilon$ permittivity and $d$ distance between plates.</p>
<p>Here some list of possible materials and their permittivity</p>
<p><img src="http://i.stack.imgur.com/oFyeM.png" alt="enter image description here"></p>
<p>The gap is 0.01m (d) so the permittivity should be more than 100 for capacitance to be more than one.
I think the capacitance must be more than one to be useful. </p>
<p>I think Schrödinger equation can be used here to simulate some things.</p>
<h2>Case</h2>
<p>Assume you have insulator as polymer which has permittivity $\epsilon = 100 000$.
Distance between plates is $d = 0.01 m$ so capacitance is $c = 1000$.</p>
<p><strong>How can you simulate the movement of electron from the one plate to another in different situations by Schrödinger equation?</strong></p> | g14432 | [
-0.053921639919281006,
0.04814928397536278,
-0.008894655853509903,
0.06074988842010498,
-0.02086373046040535,
0.008377035148441792,
-0.012223108671605587,
0.038416795432567596,
-0.0013226780574768782,
-0.015492339618504047,
-0.01506549771875143,
0.04761304333806038,
-0.0004224749864079058,
... |
<p>The string theorist Brian Greene recently came up with the book The Hidden Realities. In Chapter 9, and also in the article <a href="http://discovermagazine.com/2011/jun/03-our-universe-may-be-a-giant-hologram" rel="nofollow">http://discovermagazine.com/2011/jun/03-our-universe-may-be-a-giant-hologram</a> , he argued that our universe might be a hologram with a holographic parallel universe and compares our world to shadows in Plato's cave. Can anyone point me to where I can read up more on this/ Are we living in a black hole. Are we puppets pulled by strings from some distant surface</p> | g14433 | [
-0.022560685873031616,
0.03228322044014931,
0.01775580272078514,
-0.06758284568786621,
-0.020576555281877518,
0.061831701546907425,
0.0105925677344203,
0.04588865488767624,
0.025268690660595894,
0.024263229221105576,
0.033066362142562866,
0.0039623635821044445,
-0.0025709285400807858,
-0.0... |
<p>I am an electronics and communication engineer, specializing in signal processing. I have some touch with the mathematics concerning communication systems and also with signal processing. I want to utilize this knowledge to study and understand Quantum Mechanics from the perspective of an engineer. I am not interested in reading about the historical development of QM and i am also not interested in the particle formalism. I know things have started from the wave-particle duality but my current interests are not study QM from that angle. What I am interested is to start studying a treatment of QM from very abstract notions such as, 'what is an observable ? (without referring to any particular physical system)' and 'what is meant by incompatibility observables ?' and then go on with what is a state vector and its mathematical properties. I am okay to deal with the mathematics and abstract notions but I some how do not like the notion of a particle, velocity and momentum and such physical things as they directly contradict my intuition which is based on classical mechanics ( basic stuff and not the mathematical treatment involving phase space as i am not much aware of it).</p>
<p>I request you to give some suggestions on advantages and pitfalls in venturing into such a thing. I also request you to provide me good reference books or text books which give such a treatment of QM without assuming any previous knowledge of QM.</p> | g14434 | [
0.029743345454335213,
-0.0027783929836004972,
0.007715807296335697,
-0.04091866686940193,
0.026849176734685898,
0.028274210169911385,
-0.017780400812625885,
-0.00011154021922266111,
0.030703242868185043,
-0.0047743855975568295,
0.025638483464717865,
0.0028453459963202477,
0.02756561338901519... |
<p>A friend of mine was telling me about a storm that knocked down a power line over at their place, and it got me to thinking. Why are downed power lines dangerous? I don't see any good reason for it.</p>
<p>It seems to me that if they can put a <a href="http://en.wikipedia.org/wiki/Ground-fault_current_interrupt" rel="nofollow">GFI</a> on a hair dryer that will kill power within a fraction of a second if it falls into the sink or tub and shorts out, they ought to be able to do the same thing to a power line. Is there any reason why the basic concept can't work at that scale? Or if not, why aren't they standard equipment on power lines?</p> | g14435 | [
0.04413176327943802,
0.039611853659152985,
-0.025237033143639565,
-0.04238036647439003,
0.023414527997374535,
0.05667039379477501,
-0.010415819473564625,
0.008472551591694355,
-0.09071414172649384,
-0.08422745764255524,
0.016883285716176033,
-0.01282388437539339,
-0.004013795405626297,
-0.... |
<p>What is the phase shift of a wave that tunnels through a barrier, meaning the difference in phase between the incoming (in front of the barrier) and the outgoing (behind the barrier) waves?</p>
<p>For example, in the situations of a wave function tunneling through a classically forbidden region, where the wave is no longer oscillating, but decaying exponentially within the barrier. Or of a beam of light incident at the critical angle of an interface in a frustrated total internal reflection setup.</p> | g14436 | [
-0.007479539141058922,
0.02195788361132145,
0.0070134676061570644,
0.06045128405094147,
0.0316404290497303,
0.0068488032557070255,
0.04508727043867111,
0.026311950758099556,
-0.022376030683517456,
-0.020632173866033554,
-0.028855565935373306,
0.027503227815032005,
-0.028601262718439102,
-0... |
<p>I'm trying to understand how symmetry groups are related to potentials of the Schrodinger equation. In particular, I wish to know if it is possible to find the symmetry group of this potential
$$V(x) = A_0 +A_1x +A_2x^2 -\frac{9}{4}x^4$$</p>
<p>where $A_0$,$A_1$,$A_2$ $\in \mathbb{R}$</p>
<p>I've tried to see if it is related to the SO(3) group and the unitary group U(1), but neither seem possible. I asked this question because coming from a pure math background, I am having a really difficult time trying to understand this.</p> | g14437 | [
-0.004899939987808466,
0.021769283339381218,
-0.02220909111201763,
-0.006412873975932598,
0.029884081333875656,
0.0029040088411420584,
0.004242176655679941,
0.05137094110250473,
0.010269148275256157,
-0.01434779167175293,
-0.015971938148140907,
-0.0004736101836897433,
-0.013363504782319069,
... |
<p>I am having difficulty grasping the concept of a <a href="http://en.wikipedia.org/wiki/Longitudinal_wave" rel="nofollow">longitudinal wave</a>. My textbook definition "In longitudinal waves, the vibration is backwards and forwards in the direction of motion of the wavefront"</p>
<p>If it vibrates backwards and then forwards, would it not be in the same position it originally was?</p>
<p>Do we assume it vibrates forward at a rate faster than that at which it vibrates backwards?</p>
<p>Further, what am I supposed to 'visualize' when I think of waves, in a physics sense?</p>
<p>What is the purpose of a longitudinal wave?</p>
<p>Sorry if I am asking a lot of questions,</p> | g14438 | [
0.04887102544307709,
0.021312305703759193,
-0.006783002987504005,
-0.03134848177433014,
0.08577242493629456,
0.002241013804450631,
0.09648721665143967,
0.02304832451045513,
-0.04479823261499405,
-0.0658615455031395,
-0.009566251188516617,
0.030314015224575996,
-0.018422851338982582,
0.0083... |
<p>The Venusian atmosphere is according to CalTech's webpage <a href="http://coolcosmos.ipac.caltech.edu/ask/43-What-is-the-atmosphere-of-Venus-like-" rel="nofollow">What is the atmosphere of Venus like?</a> is 90 times heavier, than Earth's, containing predominantly carbon dioxide, some nitrogen, sulfuric acid clouds in its atmospheric chemical soup.</p>
<p>What affect would Venus' heavier atmospheric mass and composition have on how the sunset and sunrise would be viewed visually?</p> | g14439 | [
-0.005037195049226284,
-0.012600734829902649,
-0.005501909181475639,
0.04316084831953049,
-0.009938491508364677,
0.05280522629618645,
0.04182944819331169,
-0.005792250391095877,
0.011545486748218536,
-0.03685823082923889,
0.027088012546300888,
-0.02010849118232727,
0.04886745661497116,
0.0... |
<p>I was sitting in my parked car tonight when a Harley-Davidson motorcycle parked next to me. With my window down, I could hear that as the motorcycle was cooling down, it was giving off some type of “popping noises.” My first guess was that as the metal of the exhaust pipes cooled, thermal contraction occurs such that it produces some sound (even though I really have no idea why this would be true). I am hoping someone could correctly explain the cause of this popping noise. </p> | g14440 | [
0.09723006933927536,
0.028065234422683716,
0.009496876038610935,
-0.02389710023999214,
-0.004512132611125708,
0.004482737742364407,
0.010331510566174984,
0.06188468262553215,
-0.043666012585163116,
0.05619457736611366,
-0.0049330443143844604,
-0.0007460034685209394,
0.050760067999362946,
0... |
<p>For example because of newton law $F=ma$ force is acceleration times mass but if I go at a high constant velocity ($900\: \mathrm{miles/hour}$) in my car for examples I have $0$ acceleration but if I hit an obstacle by newton's law it will have no effect on me because $F=ma$ and mass of my car x 0 = 0</p>
<p>But if I do the experiment I will find that I actually hit the obstacle and it does an force on me why?</p> | g14441 | [
0.028717124834656715,
0.041174259036779404,
0.02766372263431549,
0.04678896442055702,
0.0672706589102745,
0.0479838103055954,
0.0324116088449955,
0.07678823173046112,
-0.047100938856601715,
-0.06721285730600357,
0.0011378434719517827,
-0.06066281720995903,
-0.03678814694285393,
-0.00528370... |
<p>I am trying to solve a challenging problem, and I'm hoping for some advice on how to proceed. I want to model sound waves in a cavity for the purpose of determining resonance. The plan is to answer the question numerically, but I need some better bearings regarding the physics involved.</p>
<p>Here is a summary of my main questions:</p>
<ol>
<li>How should I model this situation for a <em>closed</em> cavity?</li>
<li>What does resonance in a closed cavity even look like? (as opposed to a "whistling" kind of cavity)</li>
<li>What wave equation should I use? I'm assuming spherical for an arbitrary cavity.</li>
<li>I don't even know if using diff eq's (wave equation) is the best approach here. Would it be possible, or even advisable, to try modeling noise in the cavity and see what kind of response I get?</li>
<li>I don't want my solution to be dependent on the location of the source inside the cavity. Rather, I want to understand what the natural frequency of the space is. Is that even possible, or am I living in a fantasy here?</li>
</ol>
<p>I want to start by solving this problem for the cube, so that I understand all the parameters involved, and solve for general cavities from there. My naive guess at the solution for cubes (or rectangular prisms) is to look at standing waves in the 3 dimensions (though I don't know which harmonic would be dominant). The problem with this approach is that it does not translate to arbitrary cavities very well.</p> | g14442 | [
-0.005247707478702068,
0.021127738058567047,
-0.00179364497307688,
-0.04270390421152115,
0.013505512848496437,
-0.00330533180385828,
0.022970333695411682,
0.020265396684408188,
-0.01537728775292635,
0.007629156578332186,
-0.012817684561014175,
0.01225457526743412,
0.028004677966237068,
0.0... |
<p>I am wondering, when we change the resistance in a circuit, does the voltage tend to increase or the ampere decrease? More specifically, under what conditions does what happen?</p>
<p>E.g. If we have a high-resistance voltmeter, does increasing the resistance of the circuit itself necessary increase the reading, or might the current just decrease further?</p> | g14443 | [
0.0070184883661568165,
-0.003985208459198475,
0.0006632194854319096,
-0.02718915417790413,
0.05512723699212074,
-0.003796595148742199,
-0.0018884307937696576,
0.013212143443524837,
-0.01353716291487217,
0.0561559833586216,
-0.033095892518758774,
0.030308710411190987,
-0.036402858793735504,
... |
<p>What is the reference spectrum? I need to know how to calculate the reference spectrum of a wavelength 500nm. </p> | g14444 | [
-0.02433144487440586,
-0.04740520939230919,
0.005409639328718185,
-0.03823433816432953,
0.027071721851825714,
-0.001552727771922946,
-0.014401097781956196,
0.01746116578578949,
0.023529307916760445,
-0.00977406371384859,
0.007184990681707859,
0.07694029062986374,
-0.02248552069067955,
0.03... |
<p>Why adding just one electron changes tremendously the ionization potential from any of the nobel atoms ? </p>
<p>If it is the screening why adding a second electron increases the ionization potential ?
<a href="http://en.wikipedia.org/wiki/File:IonizationEnergyAtomicWeight.PNG" rel="nofollow">http://en.wikipedia.org/wiki/File:IonizationEnergyAtomicWeight.PNG</a></p> | g14445 | [
0.027409227564930916,
0.07526174932718277,
0.00791878066956997,
0.02210504189133644,
0.0476432666182518,
0.039598867297172546,
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0.07504373043775558,
-0.005558422766625881,
0.013438460417091846,
0.012667510658502579,
0.042149994522333145,
0.014808330684900284,
0.0145375... |
<p>The higher the quantum number(energy levels)m the higher the energy.
What does the energy refers to? Kinetic energy, potential energy, or the total mechanic energy?</p> | g14446 | [
0.05580447241663933,
0.027812447398900986,
-0.01433161273598671,
0.007924556732177734,
0.0040293289348483086,
-0.003528158413246274,
0.0055519104935228825,
0.03653744235634804,
-0.02081155776977539,
-0.0005586506449617445,
-0.01549998763948679,
-0.022276496514678,
0.0016416047001257539,
-0... |
<p>For a localized charge distribution the potential is set to zero far away from the charge distribution (at infinity)</p>
<p>Now, when grounding a conductor, i.e. connecting it to Earth, it is said that we are setting its potential to zero. </p>
<p>Why Earth's potential is zero?
And if it is zero, is it zero even if the potential is still zero at infinity?</p> | g14447 | [
0.08310984820127487,
0.0430767759680748,
0.016259385272860527,
-0.035710159689188004,
0.059166859835386276,
0.0667543113231659,
0.027801308780908585,
0.06941691040992737,
-0.06669781357049942,
-0.006199527531862259,
-0.05836859717965126,
-0.01600991189479828,
-0.02739669382572174,
-0.00243... |
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